Re: How to add 256 byte sized opaque mode in gcc

On Wed, 19 Aug 2026, Avinash Jayakar wrote:

> Hi Jakub/Richard,
> 
> Thank you for these suggestions and review.
> 
> I have attached the proposed patch based on your input to dynamically
> size the mode_unit_size array based on maximum bytesize declared in
> machine modes.
> 
> Please do let me know if any tests are required for this before sending
> to gcc-patches?

+  unsigned int max_type_size = 0;
+  for_all_modes (c, m)
+    {
+      if (max_type_size < m->bytesize)
+       max_type_size = m->bytesize;
+    }
+  if (max_type_size < (1 << 8))
+    puts ("#define MODE_UNIT_SIZE_TYPE unsigned char\n");
+  else if (max_type_size < (1 << 16))
+    puts ("#define MODE_UNIT_SIZE_TYPE unsigned short\n");
+  else
+    puts ("#define MODE_UNIT_SIZE_TYPE unsigned int\n");
+

I think you need to account for bits_per_unit here, meaning
you want to track max_unit_size which would be
((size_t)m->bytesize * 8) / bits_per_unit

Richard.

> Also as you mentioned these lookups happen quite often in compilation,
> so introducing this new mode could potentially increase compile times
> for powerpc64le right?
> 
> Thanks and regards,
> Avinash Jayakar
> 
> On Tue, 2026-08-18 at 13:06 +0200, Jakub Jelinek via Gcc wrote:
> > On Tue, Aug 18, 2026 at 12:48:55PM +0200, Richard Biener via Gcc
> > wrote:
> > > I'd use unsigned short, it's important to not use an overly large
> > > data
> > > structure for those common lookups to reduce cache footprint.  We
> > > could possibly even dynamically size the component from genmodes?
> > 
> > Yeah, I'd prefer that, emit a typedef/macro in insn-modes.h, say
> > MODE_UNIT_SIZE_TYPE, unsigned char or unsigned short depending in
> > whether all modes fit or don't fit into 255 bytes and use it for the
> > generated array too.  Similar to how e.g. the CONST_MODE_* macros
> > are defined.
> > 
> > > > @@ -1884,6 +1885,11 @@ rs6000_hard_regno_mode_ok_uncached (int
> > > > regno, machine_mode mode)
> > > >        else
> > > >  	return 0;
> > > >      }
> > > > +  /* QDOmode needs even/odd DMR register pairs.  */
> > > > +  if (mode == QDOmode)
> > > > +    {
> > > > +      return (TARGET_DMF && DMR_REGNO_P (regno) && (regno & 1)
> > > > == 0);
> > > > +    }
> > 
> > The GCC coding style here is to avoid the {}s around a single
> > statement
> > body.
> Sure I will update this.
> > 
> > 	Jakub
> 

-- 
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
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